Some fixes & upgrades

- Added Unsemantic CSS + IE-only (successor of 960.gs) -
http://unsemantic.com/ - version 09/02/2013
- Fixed some misetyped HTML5 rules on header.tpl
- Fixed forced language on <html> in the header.tpl
- Removed meta charset from <head> cause not needed anymore on
Prestashop 1.5.x
- Removed grid_prestashop.css cause replaced by Unsematic CSS
- Removed "clearfix" rules from global.css, now is used the "clearfix"
rules from Unsemantic CSS